Micron-sized and smaller-sized metal dust is inevitably generated and accumulated in GIS. Under the action of external electric field, the small-scale dust is easy to be lifted by force and dispersed in the cavity, which is one of the main causes of air gap breakdown. Aiming at the actual demand of safe operation of GIS, a micron-nano dust dispersion detection system is built to study the influence of key factors such as dust particle size, initial weight, material, and initial position on air gap breakdown voltage. A multi-dimensional comprehensive evaluation model of micron-nano dust hazard degree is constructed based on entropy weight method. The calculation and analysis show that the most dangerous condition in the coaxial cylindrical electrode at atmospheric pressure is the aluminum dust with the size of 50 nm and an initial weight of 700 mg, and the hazard coefficient is 0.615. The application of this method can comprehensively and accurately evaluate the hazard degree of GIS operation and effectively deal with potential security hazards.